A process has been well known which comprises imagewise exposure of an internal latent image type silver halide emulsion not having been previously fogged, and then subjecting the emulsion to surface development after or while being fogged, thereby obtaining a direct positive image.
Such an internal latent image type silver halide emulsion is a silver halide photographic emulsion of the type comprising a light-sensitive nucleus mainly inside silver haldie grains wherein latent images are formed mainly inside the grains upon exposure.
In the art, various techniques have been known. Main examples of such techniques include those described in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276, 3,796,577, 1,151,363, 1,150,553, and 1,011,062.
In accordance with these known approaches, a photosensitive material can be prepared with a relatively high sensitivity for direct positive type light-sensitive materials.
The mechanism of the formation of such a direct positive image is described in detail in T. H. James, "The Theory of the Photographic Process", 4th ed., Chap. 7, pp. 182-193, and U.S. Pat. No. 3,761,276.
The inventors previously worked on and applied a process for the preparation of a direct positive light-sensitive material with a low Dmin and high contrast by incorporating thiosulfonic acid in an emulsion (JP-A-1-254,946, the term of "JP-A" as used herein means an unexamined published Japanese patent application).
A direct positive photosensitive material must exhibit a high Dmax, a low Dmin and a high contrast to be put into practical use. In particular, such a direct positive photosensitive material is subject to a drop in Dmax when an additive is incorporated therein to lower Dmin.
In accordance with the method described in the above cited JP-A-1-254,946, a direct positive photosensitive material can be obtained with a low Dmin and a high contrast. However, such a direct positive photosensitive material is disadvantageous in that it exhibits a low fresh sensitivity and a large change in the sensitivity during aging upon storage (sensitization).